In the related art, when a user's telephone apparatus requests a center apparatus to issue a provisional number, the center apparatus issues a provisional number made up of a predetermined number of digits, notifies the telephone apparatus, and, at that time, registers the provisional number and the telephone number of the telephone apparatus in a conversion table in association with each other. A telephone number distribution system has been proposed in which when the user then tells the other party the provisional number notified from the center apparatus, and the other party places a telephone call by adding a specific number before the provisional number, a telephone number corresponding to the provisional number is retrieved from the conversion table in the center apparatus and transferred to the user's telephone apparatus, allowing the telephone call from the other party to be received by the provisional number. According to this system, it is possible to receive a telephone call from the other party without telling one's own telephone number to the other party (see, for example, Patent Document 1).
Also, in recent years, moves toward building a next generation network (hereinafter referred to as NGN), in which an existing line-switching telephone network is replaced with a network based on the IP (Internet Protocol) technology, have become increasingly active. The NGN includes a bandwidth guarantee function and a security function in the network itself, and performs session control using SIP (Session Initiation Protocol).    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-261935    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2006-270800
Incidentally, the telephone number distribution system described in Patent Document 1 mentioned above has a problem in that an apparatus for transferring a telephone call is necessary, there is a constraint such that incoming using a provisional number is possible but outgoing is not possible and, moreover, no condition can be put on incoming to the provisional number, making flexible operation difficult.
Also, while the telephone number management apparatus described in Patent Document 2 allows outgoing using a provisional number and setting of connection conditions, an apparatus that performs retrieval of a provisional number and a normal number and transfer of a telephone call in a centralized manner is required, and thus there is a fear of excessive load and rigorousness of operation.